Green worked with the gaffer Matt Taylor at Exeter, where he spent 12 years in their youth development before moving to Aston Villa last summer.
Speaking to Ifollow, boss Matt Taylor was pleased to have brought Dan in.
"Dan is one of the ‘best coaches I’ve worked with. We are delighted to bring him to the football club."
"He’s come to Rotherham to have a bigger responsibility in coaching and helping get the first team where they want to be and I can’t wait to see him work with this group of players. He’s a hands-on coach and will take every player, if they are willing, to the next level."